The present invention relates to the arrangements of picture-element electrodes of a color liquid-crystal display apparatus.
The color liquid-crystal display apparatus is provided with many picture elements arranged in dot-matrix shape and a coloring means arranged corresponding to each picture element. The picture element is an element which constitutes an image, and is composed of a portion wherein electrodes disposed on two opposite substrate overlap each other, and a liquid crystal sandwiched therebetween. Each picture element is controlled through application of a picture signal corresponding to each picture element so that colors are additively mixed in accordance with the same principle as that of a color CRT, with the result that an arbitrary color image including a half tone color image may be displayed. Read "Liquid-Crystal Electronics Base and Application" (Ohm Company, 1979) or the like edited by Sasaki for details on the liquid crystal.
Many modes such as twisted nematic (TN), guest host (GH), dynamic scattering mode (DSM), phase transfer, etc. are available as an operation mode of a liquid crystal display apparatus. Particularly the TN and GH provide favorable results. In the guest host, black die is used and is operated as a so-called black shutter.
Normally additive three primary colors are selected as the colors of the coloring means. An interference filter, a color filter made of inorganic or organic dye or pigment are used in the coloring means. The coloring means may be provided on the outer face of a substrate which constitutes a liquid crystal display apparatus or on the inner face thereof. In the case of the latter, the coloring means may be provided on or below a signal electrode, a scanning electrode, a picture element electrode or a common electrode.
In the color liquid crystal display apparatus, only the spectrum region of one color of three primary colors among the spectra of the incident light can be used and the remaining region are absorbed by the coloring means. In the case of a liquid crystal operation mode using a polarizer, the light intensity to be used in further reduced by half, so that it is very dark in a reflection type mode with no illuminating means in it. Thus, a light source such as incandescent lamp, fluorescent lamp, EL (Electro-luminescence) panel or the like is provided or a means for guiding ambient light to the rear face of the liquid crystal display apparatus are provided as the illuminating means. For application into a portable appliance, it is important to improve the radiation efficiency of the light source because of the severe restriction in power capacity. To faithfully reproduce picture signals, many picture elements, i.e., many scanning lines are required. For example, a liquid crystal panel for color television use will be considered. In the NTSC system of a television broadcasting, the band width of luminance signal (Y signal) is 4 MHz, while the band width of I signal and Q signal which are color phase signals are respectively 1.5 MHz, 0.5 MHz. As the sine waves of the 0.5 MHz includes 26.5 cycle waves in the 1 effective horizontal scanning period (53 microseconds), the horizontal resolution is equivalent to 53 lines or 26.5 line pairs of bright line and dark line. According to the theorem of Shannon, the loss of the information to be contained in the original signal cannot be caused if the sampling operation is performed with a frequency twice the highest frequency of the original signal. However, it is difficult to say that the images provided when the signals sampled in that manner have been reproduced as they are visually faithful to the original signal under the in influences of aliasing. As the result of our experiment the visual satisfaction is provided when the sampling has been performed with a frequency higher than three times the highest frequency. Accordingly, to reproduce the color signal of 0.5 MHz, the information having the original signal can be reproduced almost faithfully if the number of the picture elements of the same color on the same horizontal line is 80 and more.
In a liquid crystal display apparatus provided with many picture elements, one of the following three methods is used to individually control the respective picture elements.
(1) Simple matrix
As shown graphically in FIG. 7(A), a stripe shaped electrode group is provided respectively on the two opposite substrates. They are sealed to each other so that they become normal to each other so as to constitute a liquid crystal display. Row selection signals are sequentially applied upon the row electrodes (scanning electrodes) SL disposed on one substrate. An image signal is applied in synchronous relation with a row selection signal upon the column electrodes (signal electrodes) DL disposed on the other substrate. The overlap regions (which are shown by oblique lines) between the row electrodes SL and the column electrodes DL become picture elements and the liquid crystal sandwiched between both the electrodes responses to the potential difference between them. When one of both the electrodes is divided every picture element, each of this portion is called a picture element electrode.
As the liquid crystal responses to the effective value in this method, the number of the scanning lines cannot be rendered large because of crosstalk, dynamic range.
To overcome such restriction as described hereinabove, a multiple matrix has been devised. This is a method of increasing the number of the picture elements in a scanning electrode direction, instead of increasing the number of the scanning electrodes, by deformation of the signal electrode of the simple matrix. (Read "Liquid-Crystal Electronics Base and Application" (Ohm Company, 1979) edited by Sasaki).
Liquid crystal apparatus of duplex matrix and quadruplex matrix are now put on the market or being manufactured for trial. In the duplex matrix graphically shown in FIG. 7(B), the number of the scanning electrodes SE is kept the same as before, the number of the signal electrodes DL is rendered twice, the number of the picture elements (which are shown by oblique lines), and the adjacent two rows of picture elements are simultaneously scanned.
As the shape of the signal electrodes becomes complicated to produce the narrow wiring-width portion in the multiplex matrix, the wiring portion in the multiplex matrix, the wiring resistance is likely to become high. When the wiring resistance of transparent conductive film only cannot be made sufficiently low, metallic wirings are jointly used. When the metallic wirings are used, effective picture element area reduces to make the picture face dark. Also, once the multiplex degree increases, the area of the wiring portion becomes relatively large to reduce the effective picture-element area.
Also, the following two systems which are effective when the number of the picture elements is large have been developed.
(2) Addition of nonlinear element
There is a method of adding to each picture element a nonlinear element, such as varistor, back-to-back diode, MIM (metal/insulator/metal) or the like, as an active element to suppress the crosstalk. As shown graphically in FIG. 7(C), picture element electrodes PE corresponding to the respective picture elements are provided and are connected respectively with the signal electrodes DL through the nonlinear elements NL. The scanning electrodes SL are disposed, on the opposite substrate, in a direction normal to the signal electrode DL. As shown in oblique lines, the picture element is located in the overlapped portion between the picture element PE and the scanning electrode SL.
(3) Addition of switching element
This is a method of adding a switching transistor, as an active element, to each picture element to individually drive it. As shown graphically in FIG. 7(D), a picture element electrode PE corresponding to each picture element is provided and is connected with a signal electrode DL through a switching element SW. Scanning electrodes SL are provided in a direction normal to the signal electrode DL and are connected with the gate of the switching element SW. On the other hand, a common electrode CE is provided on its opposite substrate. The picture element is provided on an overlapped portion between the picture electrode PE and the common electrode CE as shown in oblique lines. A storage capacitor is added when necessary. A driving voltage is applied and the capacitor is charged during a selection period, and the applied voltage is held by the capacitor even during a non-selection period. As the liquid crystal itself is also capacitive load and its time constant of discharge is sufficiently larger than a repeated period of the driving, the storage capacitor can be omitted. A thin film transistor, a MOS-FET formed on silicon wafer or the like is used as a switching transistor.
Although not shown concretely in the above description, a color filter is disposed corresponding to each picture element in a color liquid-crystal display apparatus.
The present invention is applicable to the above-described methods (1) through (3), and the effect is large particularly in the case of the (3).
The color arrangement which is the subject of the present invention will be described hereinafter. A color liquid-crystal display apparatus using the liquid crystal is already disclosed in such as U.S. Pat. No. 3,840,695. An XY matrix display apparatus using three primary colors of stripe-shaped color-filter is known and a matrix display apparatus with a thin film transistor (TFT) being provided per picture element electrode is disclosed in the above mentioned U.S. patent. In these examples, only the use of the three primary colors of stripe-shaped or mosaic-shaped color filter is described without any concrete mention of the three primary colors of arrangement method in the color arrangement. Also, in the conventional TFT matrix display substrate, the signal electrode and the scanning electrode which are connected with the column of the picture element and the row thereof were rectilinear, and all the picture-element electrodes were arranged on the same side of an intersecting point between the corresponding signal electrode and scanning electrode.
The conventional color arrangement in largely divided into a stripe shape and a mosaic shape. In the stripe-shaped color arrangement of the former, picture elements are arranged in parallel like longitudinal stripe (FIG. 8(A)) and lateral stripe (FIG. 8(B)). In the mosaic-shaped color arrangement of the latter, square or rectangular picture elements are arranged in a checkered shape. Nine picture-element staircase shape (FIG. 8(C)), longitudinal six picture-element type (FIG. 8(D)), lateral six picture-element type (FIG. 8(E)), four picture-element type (FIG. 8(F)), and their modifications are also taken into consideration.
Referring to FIG. 8, reference characters R, G, B respectively show the red, green, blue of additive three primary colors, and a parenthesis ( shows a basic period of the arrangement pattern of three colors (R, G, B). The six picture-element type and the four picture-element type have been proposed by the present inventors.
When picture elements sufficient in number to faithfully reproduce the picture signals are provided, the color arrangement of the picture elements does not influence the quality of the reproduced images. But when the number of picture elements is not sufficiently large, the quality of the reproduced images is adversely affected by the color arrangement. In the case of the stripe-shaped color arrangement, the color switching of the driving signal is not necessary in the longitudinal stripe and all that is necessary in the lateral stripe is to switch the color for each of the scanning lines before the analog line memory, but the spatial resolution in a direction normal to the stripe is inferior by as much as three picture elements pitch, thus causing Moire stripes easily. Also, in a condition where the white balance is filled, the brightness of the blue is so low in terms of the visibility characteristics that the blue looks extremely dark. Thus, blue lines look like black stripe patterns, thus spoiling the picture quality. On the other hand, in the case of mosaic-shaped color arrangement, in the nine picture-element staircase type, the same color of picture elements are obliquely connected in the staircase. The spatial resolution in a direction normal to its connection direction becomes 3/.sqroot.2 picture element pitch, thus reducing the above-described disadvantage somewhat, but the color switching of the picture signal is required for each of the signal electrodes and each o scanning electrodes.
To improve such disadvantages as described hereinabove, the present inventors have proposed the color arrangement of the six picture-element type and four picture-element type. In the six picture-element type, the blue lines become zigzag, which makes the stripe patterns less conspicuous. In the four picture-element type, the green picture-elements are arranged in checkered patterns, and the spatial resolution becomes one picture-element pitch both longitudinally and laterally, and becomes .sqroot.2 picture-element pitch even in the worst oblique direction, thus resulting in considerably small anisotropy. As the blue pictures are arranged separately, no dark lines are produced. The visibility characteristics are such that the space resolution with respect to brilliance (brightness) is high, but the space resolution with respect to the color difference is not so high. In the contribution towards the brilliance, the green among the red, green, and blue is largest. Accordingly, if the green picture element faithfully reproduces a brilliance signal (Y signal), the image quality is not deteriorated even if the space resolution is half as much as the green in the red and blue picture elements.
However, in the mosaic-shaped color arrangement, the resolution and the reproduced image quality are improved, but the driving circuit of the picture element is complicated. Namely, as described hereinabove, the colors of the picture element to be driven by the same signal electrode are two or three, thus requiring the color switching of the picture signal.
FIGS. 9(A), (B) are connection diagrams in a case where the liquid crystal is driven with a thin film transistor (TFT) of the conventional mosaic-shaped picture element arrangement. All the picture-element electrodes are arranged on the same side of an intersecting point between the corresponding signal electrode and the scanning electrode. Drain electrode of TFT 1 . . . and the storage capacitors which are provided when necessary are connected with the picture element electrodes arranged in mosaic shape (not shown). In the drawing, the capacitors 2 . . . are equivalent circuits showing the capacity of the liquid crystal and the point of each arrow is connected commonly to a common electrode. Each scanning electrode (gate line) 3 is connected with the gates of the TFT 1 . . . arranged laterally. Also, each of the signal electrodes (data line) 4 is connected with the source electrodes of the TFT 1 arranged longitudinally. A gate driver 5 composed of a shift register sequentially scans the scanning electrodes 3 periodically with scanning pulses (horizontal synchronizing signal H) to turn on the TFT 1 . . . connected to the selected scanning electrode 3. Video signals are applied upon the signal electrodes 4 . . . , as described hereinabove, in synchronous relation, and are applied upon picture-element electrodes and capacitors 2 . . . (not shown) through the TFT 1 . . . to drive the liquid crystal. The capacitors 2 maintain the voltage to be applied upon the liquid crystal during a time period with the TFT 1 . . . off. When the time constant of the liquid crystal is sufficiently large as compared with the scanning period, the storage capacitor is not required to be provided.
In the conventional mosaic-shaped color arrangement, picture elements of two colors or three colors are connected to the same signal electrode 4 to apply a signal (red, green, blue) corresponding to each color to a signal electrode 4. Each picture element selects only the corresponding signal through the periodic scanning operation of the scanning electrodes 3 . . . . As a result, a circuit which periodically switches the signals (red, green, blue) is required. In FIG. 6(A), each picture signal of the Red, Green, Blue is respectively inputted into analog line memories so that the picture signal of each color sampled is inputted into a color switching circuit 7 provided for each of the signal electrodes 4, thereby selecting a signal corresponding to the color arrangement. Here, the analog line memory 6 samples a color picture-element R, G or B to output a signal in synchronous relation to the scanning electrode. In FIG. 6(B), before the picture signal is inputted to an analog line memory 6, each picture signal of the R, G, B is time-shared in accordance with the color arrangement by a color switching 7, is converted into a serial signal and is sent into an analog line memory 6.
In the conventional mosaic-shaped color arrangement, all the picture-element electrodes are arranged on the same side of the intersecting points between the corresponding signal electrodes and scanning electrodes, and thus two colors or three colors of picture-element electrodes are connected with the same signal electrode. For example, in the case of a lateral six picture-element type arrangement, the color of a picture element to be driven by the same signal electrode alternately changes like RGRG . . . , GBGB . . . , BRBR . . . every one signal electrode. Accordingly, a color switching circuit for the signals of the analog line memory becomes necessary, thus requiring a complicated driving circuit.